Whip them as hard as you feel is necessary.
So long as you don't leave marks. That's why you do it through clothing, and with thin objects. Although, they do leave red marks.
Didn't do the dishes? Smack them until they are done.
Skipped school? Give them a couple whacks for every hour they will miss, and some for lying and etc.
Got a chick pregnant? A thumbs up followed by a hard smacking on the ass.
That's my mothers opinion.
I don't think physical punishments are right unless it was harmful to someone else as well.
Ex. Your son gives a kid in school a bloody nose. A couple spanking followed by a non-physical punishment is what I feel would be necessary, unless he was standing up for himself. You gotta do it or else your life is living hell. :o